The major goal of this project is to determine the teratogenic potential of certain lipophilic drugs selected according to their strong growth-inhibition of cultured mammalian cells. New methods permitting the cultivation of early somite-stage embryos have been adopted to avoid the difficulties of absorption, metabolism, and excretion encountered in whole animals. We found that the anticonvulsants valproic acid (VPA) and diphenylhydantoin (DPH) cause developmental defects in a dose-dependent manner. The frequency of defects increased slightly when the developing embryo was examined by more refined methods. A secondary goal was to evaluate the utility of the embryo culture system for developmental biology studies; several pilot experiments were undertaken.